Hardfacing materials are applied to a variety of downhole tools to improve wear resistance. Hardfacing may be used in an effort to improve both the hardness and fracture toughness of the downhole tool. Composite materials have been applied to the surfaces of downhole tools, in particular drill bits that are subjected to extreme wear. These composite or hard particle materials are often referred to as “hardfacing” materials and typically include at least one phase that exhibits relatively high hardness and another phase that exhibits relatively high fracture toughness. For example, a typical hardfacing material may include tungsten carbide particles substantially randomly dispersed throughout an iron-based matrix material. The tungsten carbide particles exhibit relatively high hardness, while the matrix material exhibits relatively high fracture toughness.
An example of downhole tools which may have hardfacing compositions applied thereon are bits for drilling oil wells. Drill bits used to drill wellbores through earthen formations generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, which usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or ultra hard materials, including, for example, natural or synthetic diamond, boron nitride, and tungsten carbide.
Drill bits of the second category are typically referred to as “roller cone” bits, which include a bit body having one or more roller cones rotatably mounted to the bit body. The bit body is typically formed from steel or another high strength material and includes a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include “insert” cutting elements that are press (interference) fit into holes formed and/or machined into the roller cones, referred to herein as “insert” roller cone bits. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or ultra hard materials.
Milled tooth bits include one or more roller cones rotatably mounted to a bit body. The one or more roller cones are typically made from steel and include a plurality of teeth formed integrally with the material from which the roller cones are made. Typically, a hardfacing material is applied to the exterior surface of the teeth to improve the wear resistance of the teeth. The hardfacing material typically includes one or more metal carbides, which are bonded to the steel teeth by a metal alloy (“binder alloy”). Once applied, the carbide particles are in effect suspended in a matrix of metal forming a layer on the surface. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal provides fracture toughness to the hardfacing.
Many factors affect the durability of a hardfacing composition in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and the matrix metal or alloy.
The metal carbide most commonly used in hardfacing is tungsten carbide. Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Three types of tungsten carbide commonly used in hardfacing drill bits are cast tungsten carbide, mono-tungsten carbide, and sintered tungsten carbide (also known as cemented tungsten carbide).
Tungsten generally forms two carbides, monotungsten carbide (WC) and ditungsten carbide (W2C). Cast carbide is a eutectic mixture of the WC and W2C compounds, as such the carbon content in cast carbide is sub-stoichiometric, (i.e., it has less carbon than the monotungsten carbide). Cast carbide is typically made by resistance heating tungsten in contact with carbon in a graphite crucible having a hole through which the resultant eutectic mixture drips. The liquid is quenched in a bath of oil and is subsequently comminuted to the desired particle size and shape.
Monotungsten carbide is essentially stoichiometric tungsten carbide. One type of monotungsten carbide is macro-crystalline tungsten carbide. Macro-crystalline tungsten carbide may be formed using a high temperature thermite process during which ore concentrate is converted directly to monotungsten carbide.
Another type of monotungsten carbide is carburized tungsten carbide which is typically multicrystalline in form, i.e., composed of tungsten carbide agglomerates. Carburized tungsten carbide may be formed using a carburization process where solid-state diffusion of carbon into tungsten metal occurs to produce monotungsten carbide. Typical monotungsten carbide contains a minimum of 99.8% by weight of tungsten carbide with a total carbon content in the range of from about 6.08% to about 6.18% by weight, preferably about 6.13% by weight, based on the weight of tungsten carbide.
Sintered tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with a metal binder such as cobalt. Sintered tungsten carbide may be produced by mixing an organic wax, monotungsten carbide and metal binder; pressing the mixture to form a green compact; sintering the green compact at temperatures near the melting point of the metal binder; and comminuting the resulting sintered compact to form particles of the desired particle size and shape.
As mentioned above, conventional hardfacing of milled tooth bits usually comprises particles of tungsten carbide that are bonded to the steel teeth using a metal binder alloy. Most hardfacing on drilling bits uses steel or Ni based alloys as the metal binders, although other alloys may also be used.
A typical technique for applying hardfacing to a downhole tool such as a drilling bit is by oxyacetylene welding. A welding “rod” or stick may be formed of a tube of mild steel sheet enclosing a filler (carbide phase) which is primarily carbide particles. The carbide phase may also include deoxidizer for the steel, flux, and a resin binder to retain the particles in the tube during welding. The hardfacing is applied by melting the rod on the surface of the tool. The steel tube melts to weld to the surface and provides the matrix for the carbide particles in the hardfacing. During application, the deoxidizer alloys with the mild steel of the tube.
Although a mild steel sheet may be used when forming the tubes, the steel in the hardfacing as applied to a tool is a hard, wear resistant, alloy steel. This occurs through the mixing of other elements with the mild steel during welding.
It is particularly important to provide as much wear resistance and toughness as possible on the teeth of a rock bit cutter cone. Wear resistance is meant to include abrasion resistance, and/or erosion resistance. The effective life of the cone is enhanced as wear and fracture resistance of the hardfacing is increased. It is desirable to keep the teeth protruding as far as possible from the body of the cone since the rate of penetration of the bit into the rock formation is enhanced by maintaining longer teeth. As wear occurs on the teeth, they get shorter and the drill bit may be replaced when the rate of penetration decreases to an unacceptable level. It is desirable to minimize wear so that the footage drilled by each bit is maximized. This not only decreases direct cost, but also decreases the frequency of having to “round trip” a drill string to replace a worn bit with a new one.
One wear mechanism of the hardfacing material during drilling is abrasion wear. This is typically the dominant wear mechanism on the outer row of teeth on the cutter cone, also referred to as the heel or gage row (other rows of teeth are referred to as “inner rows”). This wear occurs as the teeth rub against the wall or “gage” of the borehole being drilled. Similar abrasion wear occurs on the flank and inner side surfaces of the teeth where drill cuttings run between the teeth.
A hardfacing material having a low toughness can experience flaking or chipping of the hardfacing material. Flaking or chipping of the hardfacing material on the crest of the teeth of the inner and gage rows can lead to cratering of the hardfacing material which can dramatically reduce the life of the bit. Chipping and flaking of the hardfacing result from fracture in the matrix and the carbide particles. Local chipping of the matrix surrounding the carbide particles may result in the dislodging, or pull-out, of the carbide particles which is responsible for cratering in the hardfacing material. Cratering results in a substantial loss of the hardfacing material during drilling which can lead to exposure of the relatively soft base metal of the teeth and subsequent rapid wear. As a result, the drilling efficiency is greatly reduced. Therefore, in addition to improving the wear resistance or hardness of the hardfacing material, it is also important to improve the toughness of the matrix and the carbide particles, especially at the crest of the teeth.
Thus, advances in wear resistance and toughness of hardfacing are desirable to enhance the durability of downhole tools, for example the footage a drill bit can drill before becoming dull and to enhance the rate of penetration of such drill bits. Such improvements translate directly into a reduction of drilling expenses. The composition of a hardfacing material and microstructure of the hardfacing material applied to the surfaces of a downhole tool, in particular a drill bit, are related to the degree of wear resistance and toughness. It is desirable to have a composition of hardfacing material that, when applied to wear surfaces, provides improved wear resistance and toughness.